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Wafer-Fused AlGaAs/GaAs/GaN Heterojunction Bipolar Transistors

thesis
posted on 2009-04-17, 00:00 authored by Chuanxin Lian
(Al)GaN materials have established themselves in high speed and high power ap-plications, owing to their large band gap energies and high anticipated electron saturation velocities. Unlike the rapid development of AlGaN/GaN high electron mobility transistors (HEMTs), that of AlGaN/GaN heterojunction bipolar transistors (HBTs) has been greatly impeded by the highly resistive p-GaN material. A proposed solution is to utilize AlGaAs/GaAs heterostructures as the emitter-base and keep GaN as the collector. Com-bining the unity emitter injection efficiency and the large base transport factor of Al-GaAs/GaAs, and the high breakdown field of GaN, an AlGaAs/GaAs/GaN npn HBT is expected to operate at high speed and high power densities. Unfortunately, the large lat-tice mismatch between GaAs and GaN makes it very difficult to epitaxially grow high quality GaAs on GaN. Instead, direct wafer fusion has been used to heterogeneously in-tegrate AlGaAs/GaAs and GaN for HBT fabrication. Encouraging device performance has been achieved in wafer-fused AlGaAs/GaAs/GaN HBTs, demonstrating the feasibili-ty of forming active device regions by direct wafer fusion. By optimizing the wafer fusion and substrate removal process, smooth and conti-nuous (Al)GaAs films have been obtained on GaN. The fused p+-GaAs-GaN hetero-junctions have been characterized by electrical measurements. A large number of interface states and an electron energy barrier may exist at the fused GaAs/GaN interface resulting in adverse electron blocking effects at the base-collector junction. A broad range of fusion conditions have been explored and current gain as high as 20 has been achieved in an AlGaAs/GaAs/GaN HBT fused at 450 Ìâå¼C for 2 h. The fused HBTs were found to exhibit larger breakdown voltages than the as-grown AlGaAs/GaAs/GaAs HBTs. The first wafer-fused RF HBT has been fabricated and characterized as well. Current gain cutoff frequency (fT) of 2.6 GHz and maximum oscillation frequency (fmax) of 1.0 GHz have been obtained in a self-aligned AlGaAs/GaAs/GaN HBT with an emitter area of 5 ÌÄ' 7 Ì_å_m2. Finally, various mechanisms limiting the fused HBT performance have been ex-amined and the fused GaAs/GaN interface is believed to be the dominant factor govern-ing the fused device characteristics.

History

Date Modified

2017-06-05

Defense Date

2009-03-25

Research Director(s)

Huili Grace Xing

Committee Members

Alan Seabaugh Greg Snider Patrick Fay

Degree

  • Doctor of Philosophy

Degree Level

  • Doctoral Dissertation

Language

  • English

Alternate Identifier

etd-04172009-024315

Publisher

University of Notre Dame

Program Name

  • Electrical Engineering

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